The present invention relates to a laser recorder using a semiconductor laser which is capable of reproducing an image such as a picture having half-tones with a high quality.
In intensity-modulating a laser beam for recording an image having half-tones (hereinafter referred to as "a half-tone image" when applicable), any of (1) a technique of using an ultrasonic optical modulator, (2) a technique of varying the discharge current of a gas laser, and (3) a technique of varying the current of a semiconductor laser may be employed.
The first technique is disadvantageous in that it is expensive and requires an intricate construction because of the need for an expensive ultrasonic optical modulator and a fine adjustment mechanism for matching Bragg angles in the modulator.
The second technique is also disadvantageous in that the modulating frequency is in a low frequency band of the order of several hundreds of Hertz and the service life of the laser tube is reduced by varying the discharge current.
The third technique wherein the current of a semiconductor laser is varied suffers from the drawback that, since the optical output vs. current characteristic curve of the semiconductor laser is as shown in FIG. 1, the optical output is greatly varied merely by slightly changing the input current. Therefore it is considerably difficult to record a half-tone image by controlling the optical output in an analog mode by varying the input current.
Accordingly, an object of the invention is to provide a laser recorder which can produce a half-tone image with a high accuracy.
In the laser recorder of the invention, the high frequency modulation characteristic of a semiconductor laser is utilized to provide several tens to several hundreds of modulation levels utilizing binary modulation.
A method wherein an input signal is sampled with a sampling pulse, and a high frequency pulse signal having a frequency of at least 10 Hz is produced using the sampling pulse so that the number of high frequency pulses outputted in each sampling period is controlled according to the input signal with the pulses applied to a semiconductor laser thereby to record a half-tone image has been disclosed in United States patent application No. 214,815 filed Dec. 9, 1980 (corresponding to Japanese Patent application No. 168565/1979) filed by the present applicant. The method therein described is referred to as "a pulse number modulation method" hereinafter.
The term "sampling pulse" as used herein is intended to mean a pulse for sampling an input video signal at predetermined time intervals. The frequency of the sampling pulse can be selected as desired. However, it is preferable that it be slightly higher than the maximum frequency of the video signal in order to reproduce the image with a high resolution. Furthermore, the term "high frequency pulse" is intended to mean a pulse whose frequency is higher than that of the sampling pulse mentioned above. Preferably, the frequency of the high frequency pulse signal is several hundred to several thousand times that of the sampling pulse signal. These two pulse signals may be generated separately although it is preferable that the sampling pulse signal be obtained by subjecting the high frequency pulse signal to frequency division.
The amount of exposure of each of the picture elements which form an image is determined by the number of high-frequency pulses which are applied to a semiconductor laser with the number of pulses being determined according to the level of an input video signal during the respective sampling period. That is, the total optical energy applied to a picture element, i.e., the exposure E, is defined by the following expression: EQU E=N.multidot..DELTA.e (1)
where .DELTA.e is the optical energy which is applied to a photo-sensitive material by the semiconductor laser in response to one high frequency pulse and N is the number of high-frequency pulses (pulse number) which are provided according to the level of an input video signal for the picture element.
The high-frequency pulse number N not only corresponds linearly to the input signal but also includes a logarithmic conversion relation, a recording material characteristic, or an input/output characteristic stored. The input signal is a video signal which may be an analog signal or a digital signal.
The relation between pulse numbers and densities of an image to be recorded in the case where the image is recorded by a semiconductor laser which is controlled by the number of high-frequency pulses will be described with reference to FIG. 2.
In FIG. 2, a curve I is an example of the characteristic curve of a recording material, indicating the logarithm of the exposure amount E with density, and a curve II is an example of the relation between the numbers of high-frequency pulses and the logarithms of exposure amounts E of the recording material which are determined from the numbers.
In FIG. 2, once a density level has been selected, the corresponding high-frequency pulse number N can be obtained by following the arrow. When the density level D is changed from 0.1 to 0.2 in the low density range, the high-frequency pulse number N increases by only about nine. On the other hand, when the density level D is changed from 1.3 to 1.4 in the high density range, the high-frequency pulse number increases by about fifty pulses.
As is apparent from the above description, in order to reproduce the gradations of an image with a sufficiently high accuracy at equal density intervals, the frequency of the high frequency pulse must be much higher than that of the sampling pulse, for instance, several hundred times or, if necessary, several thousand times higher.
The relation of the sampling pulse frequency f.sub.s, the high-frequency pulse frequency f.sub.H, and the maximum level of the input signal, i.e., the maximum pulse number N.sub.max which is required for the level of the input signal to which the maximum exposure corresponds is: EQU f.sub.H .gtoreq.N.sub.max .times.f.sub.s ( 2)
The maximum pulse number N.sub.max will be larger than the values specified in FIG. 2 if the density intervals are sufficiently small to reproduce the image with a high accuracy or for certain ranges of the characteristic of the photosensitive material such as .gamma. (the maximum gradient of the characteristic curve) and a range of density D. As a result, the high-frequency pulse frequency is greatly increased making it difficult to provide circuitry implementing the above-described modulation method.
By way of example, if the sampling frequency f.sub.s =100 KH.sub.z, and the maximum pulse number N.sub.max =500, the corresponding necessary high-frequency pulse frequency f.sub.H is: EQU f.sub.H .gtoreq.N.sub.max .times.f.sub.s =50 MHz.
Accordingly, a circuit for practicing the above-described modulation method cannot be constructed with standard TTL (transistor-transistor-logic) elements. Thus, the conventional modulation method is disadvantageous in that ECL (emitter-coupled logic) elements or the like must be used to implement the required circuit and hence the circuit has a considerably high manufacturing cost.
Furthermore, another disadvantage is that, even if the maximum number N.sub.max of high-frequency pulses is outputted during the sampling period of picture element, no light is outputted between the pulses as indicated by shaded portions in FIG. 3B as a result of which the efficiency of utilization of light is low with respect to the quantity of light produced by the light source making it necessary to increase the quantity of output light provided by the light source.
Moreover, if an optical output is provided with a pulse width corresponding to a high-frequency pulse number N as shown in FIG. 3C (hereinafter referred to as "pulse width modulation" when applicable), the efficiency of utilization of light is about doubled and the exposure .DELTA.e (cf. FIG. 3C) is also about double that for pulse number modulation, as a result of which the exposure level resolution of each picture element, i.e., the reproduction density resolution, is necessarily decreased.
In view of the foregoing, an object of the invention is to decrease the high-frequency pulse frequency f.sub.H to half of that used in the conventional method without decreasing the reproduction density resolution thereby to decrease the circuit cost and to substantially double the efficiency of utilization of the light of the light source.